The present invention relates to friction clutches in general, especially to friction clutches for use in motor vehicles, and more particularly to improvements in friction clutches of the type wherein certain components are non-rotatably but axially movably coupled to each other by leaf springs or other deformable coupling means.
It is well known to construct a friction clutch in such a way that a diaphragm spring or another suitable energy storing device urges a pressure plate against the friction lining of a clutch plate or disc and that the pressure plate is axially movably but non-rotatably secured to the clutch housing, e.g., to a cover of the housing, by several equidistant sets of leaf springs which are deformable in directions to allow for axial movements of the pressure plate under or against the bias of the diaphragm spring. Reference may be had, for example, to German Offenlegungsschrift No. 1,600,093 which discloses a friction clutch of the just outlined character. In the clutch of this German publication, the pressure plate is provided with equidistant projections (as considered in the circumferential direction of the clutch) which extend radially outwardly and each of which is connected with one end portion of a leaf spring by means of a rivet. The other end portions of the leaf springs are connected to the housing of the friction clutch. The orientation of the leaf springs is such that they allow the pressure plate to move axially of the housing but the pressure plate and the housing are held against angular movement with reference to one another. The shanks of the rivets extend through openings which are provided therefor in the respective end portions of the leaf springs. The shank of each rivet also extends through a hole in the respective projection of the pressure plate, and the rivets have heads adjacent to those sides of the projections which face away from the respective leaf springs. The space requirements of the rivet heads at the exterior of the pressure plate contribute to the axial dimensions of the friction clutch. Moreover, the projections (which are provided for the express purpose of supporting the respective end portions of the leaf springs) contribute to the dimensions of the friction clutch, as considered in the radial direction of the clutch housing. This creates problems in many types of compact vehicles where the space is at a premium and, moreover, the projections of the pressure plate contribute to the weight of the clutch. Still further, centrifugal force acting upon the projections renders it necessary to increase the weight, mass and bulk of the circular main portion of the pressure plate so that such main portion can stand the developing stresses even if the pressure plate is rotated at a very high speed. The standards regarding the bursting strength of pressure plates in friction clutches are very high and must be met by the component parts of friction clutches which are intended for use in motor vehicles and the like.
Another mode of securing the end portions of leaf springs to an axially movable pressure plate in a friction clutch is disclosed in U.S. Pat. No. 3,283,864 granted Nov. 8, 1966 to Motsch. The fastening means comprises rivets extending through those portions of the pressure plate which bear against the clutch disc when the clutch is to transmit torque. To this end, the pressure plate is formed with an annulus of through holes in the region of its friction generating surface, and each hole has an enlarged portion at that end which terminates in the friction surface. The shank of each rivet extends through the opening of the respective leaf spring and through the smaller-diameter portion of the respective through hole in the pressure plate, and each rivet has a head in the larger-diameter portion of the respective hole. A drawback of such proposal is that the making of holes in the annular portion of the pressure plate reduces the area of the friction surface (namely, of the surface which engages the adjacent lining of the clutch disc when the clutch is engaged), especially since the larger-diameter ends of such holes terminate at the friction surface. This means that the remaining portion of the friction surface is subjected to more pronounced wear, especially in the regions between the neighboring holes, and the friction surface is likely to develop pronounced channels or furrows after a relatively short period of use. The development of furrows in the friction surface adversely influences the torque transmitting action because it affects the frictional engagement between such surface and the adjacent lining of the clutch disc. Still further, weakening of the pressure plate due to the provision of through holes in the friction surface entails localized accumulations of heat with attendant development of internal stresses and distortion of the pressure plate. The distortion can be so pronounced that it prevents disengagement of the pressure plate from the clutch disc and/or delays such disengagement and/or allows for mere partial disengagement of such parts from one another. Moreover, localized accumulations of heat in the regions of through holes are likely to entail the development of cracks which, in turn, can cause the pressure plate to burst within the higher RPM range.
A further mode of fastening leaf springs to the pressure plate of a friction clutch is disclosed in German Utility Model No. 1,925,053. The fastening means includes hollow screws which extend into tapped bores of the pressure plate. A drawback of such proposal is that vibrations, which are bound to develop when the friction clutch is in use in a motor vehicle or the like, cause the screws to become loose and to thus put not only the clutch but also the entire vehicle out of commission. Loosening of screws in the pressure plate is believed to be attributable to the absence of accurate mesh between the external threads of the screws and the internal threads of the pressure plate, i.e., to the absence of reliable and uniform transmission of forces between the pressure plate and the springs. This results in overstressing and excessive wear upon certain portions of the windings of the threads with attendant loosening of the screws and a breakdown of the connections between the leaf springs and the pressure plate. A further drawback of the just described fastening means is that the dimensions of openings in the leaf springs must be selected with utmost accuracy in order to prevent wobbling of the leaf springs with reference to the shanks of the screws. Any wobbling of the leaf springs would even further increase the likelihood of rapid loosening of the screws and would also result in pronounced wear upon the material of the leaf springs around the respective openings. This would entail immediate or rapid separation of leaf springs from the pressure plate.